A silica glass is used for a lens, a prism and a photomask of a photolithography instrument in manufacturing of a large-scale integrated circuit (LSI), for a TFT substrate used for a display, for a tube of a lamp, for a window material, for a reflection plate, for a cleaning container in a semiconductor industry, for a container for melting of a silicon semiconductor, and so forth. However, an expensive compound such as silicon tetrachloride must be used as a raw material for these silica glasses; on top of that, melting temperature and processing temperature of a silica glass is extraordinary high, as high as about 2000° C., thereby leading to a high energy consumption and a high cost. Accordingly, from the past, a method for producing a silica glass by using a relatively inexpensive, powdered raw material has been considered.
For example, in Patent Document 1, a method (slip casting method), wherein at least two different kinds of silica glass particles, for example, silica glass fine particles and silica glass granules are mixed to obtain a water-containing suspension solution, which is then press molded and sintered at high temperature to obtain a silica-containing composite body, is disclosed. In Patent Document 2, a method, wherein a mixed solution (slurry) containing silica glass particles having the size of 100 μm or less and silica glass granules having the size of 100 μm or more is prepared, then the slurry is cast into a molding frame, dried, and then sintered to obtain an opaque silica glass composite material, is disclosed. In these conventional slip casting methods, however, shrinkage of a molded article in a drying process and a sintering process is so significant that a thick silica glass article with a high dimensional precision could not be obtained.
Accordingly, there are problems in each method for producing a silica glass article as mentioned above. Therefore, as a method for producing a silica crucible for manufacturing of a single crystal silicon used for LSI (for a device), such production methods as those disclosed in Patent Document 3 and Patent Document 4 are being used still today. In these methods, after a powdered, ultra-highly purified natural quartz or a powdered synthetic cristobalite is fed into a rotating frame and then molded, carbon electrodes are inserted from the top and then electrically charged, thereby causing arc discharge to raise the atmospheric temperature to a temperature range for melting of the powdered quartz (temperature is estimated in the range from about 1800 to about 2100° C.) so that the powdered raw quartz may be melted and sintered.
In the methods such as those mentioned above, however, there has been a problem of a high cost because a powdered raw material quartz with high purity is used. In addition, because various kinds of impure gases are dissolved in a produced silica crucible, the gases are released and then incorporated into a silicon single crystal as gaseous bubbles thereby causing such problems as defects called a void and a pinhole when it is used as a silica crucible for growing of a silicon single crystal; and thus this has been causing problems in production cost as well as quality of the silicon crystal. In addition, there has been a big problem in durability of the silica crucible because of low etching resistance to a silicon melt at the time of pulling up of a single crystal silicon.
A method to improve the etching resistance to a silicon melt in a silica crucible for pulling up of a single crystal is shown in Patent Document 5. In Patent Document 5, an effect of applying a crystallization accelerator on an inner surface of a silica glass crucible is shown. As the crystallization accelerator, Mg, Sr, Ca, and Ba, which are alkaline earth metal elements belonging to the 2a group, and Al, which is the element belonging to the 3b group, are shown. However, a silica glass crucible as shown in Patent Document 5 was not the one having a transparent silica glass layer completely free from gaseous bubbles in an inner surface part of the crucible, but the one containing micro gaseous bubbles and inhomogeneously undissolved particles of various doped elements. Accordingly, there have been problems frequently that a pulled-up silicon single crystal contains silica fine particles as foreign substances and has defects such as a void and a pinhole. In addition, there appeared a problem of deformation of a crucible inner surface caused by large expansion of micro gaseous bubbles present inside the crucible during pulling up of a silicon single crystal.
A method to reduce gaseous bubbles in a silica glass in an inner surface of a silica crucible used for pulling up of a single crystal so that bubble expansion of the silica crucible in use may be suppressed is described in Patent Document 6. In Patent Document 6, it is disclosed that a silica crucible inner surface having few gaseous bubbles can be obtained if a powdered raw material for the silica crucible is made to contain hydrogen molecules with the concentration of 5×1017 to 3×1019 molecules/g. However, with this method, although amount of gaseous bubbles in the silica crucible inner surface could be reduced, an etching resistance to a silicon melt could not be improved by crystallizing the silica crucible inner surface to cristobalite. In addition, there has been a problem of a poor storage property of a hydrogen-containing powdered raw material because hydrogen molecules doped in the powdered raw material are gradually released to outside during storage of the powdered raw material.
A method for reducing growth of gaseous bubbles during the time that a silica crucible for silicon pulling up is in use is described in Patent Document 7. In this document, a method is disclosed wherein inside of a molded container is made an atmosphere of a helium gas or a hydrogen gas while degassing from its outside by aspiration during arc discharge melting by carbon electrodes in manufacturing of a crucible. With this method, however, even though amount of gaseous bubbles in the silica glass inner surface layer could be reduced, the OH group concentration in the silica crucible could not be reduced to a certain controlled level, nor was possible to improve durability and heat resistance of the silica crucible by finely crystallizing the inner surface to cristobalite during the time of using the crucible.
In Patent Document 8, a method for reducing amount of gaseous bubbles contained in a silica crucible used for pulling up of a silicon crystal is disclosed. In the Document, it is disclosed that any of a hydrogen gas and a helium gas or both is supplied to a powder article of the container during heating at the time of crucible production.
In Patent Document 9, it is disclosed that, during heating at the time of crucible production, an arc melting is started and continued after a helium gas or an argon gas is supplied to a powder article of the container; and before termination of the arc melting, supply of a helium gas or an argon gas is stopped or the amount thereof is reduced, while supply of a hydrogen gas is started.
In these methods, however, in similar to the foregoing, even though gaseous bubbles inside the silica crucible could be reduced, durability and heat resistance of the crucible could not be improved by protecting the inner surface by finely crystallizing the inner surface to cristobalite during the time of using the silica crucible.